This invention generally relates to RF (Radio Frequency) switching capability, and, in particular, to controlling impedance matching in an RF matching network using solid state elements.
Some plasma ion sources are driven by an RF power source that ionizes a gas to create a plasma. However, the plasma presents a complex impedance to the power source. Therefore, RF matching networks are commonly designed to match the complex impedance of the plasma so as to present a resistive load to the power source. For example, in a typical plasma ion source, an RF matching network can convert a complex impedance of a plasma at 13.56 MHz to a 50-Ohm resistive load for the generator. It should also be understood that such RF matching networks are employed to match other loads that present a complex impedance, such as transmission lines, filter networks, signal-processing circuits, and free-space antenna systems.
In certain plasma applications, an RF matching network runs in at least two modes: (1) a START mode, when no plasma initially exists in the plasma chamber; and (2) a RUN mode, after the plasma has been ignited. The two modes present dramatically different loads to the RF power generator. Therefore, the load capacitance of the matching network must change dramatically and quickly when the plasma is ignited.
In one approach, the load capacitance of the matching network is modified by a mechanical drive that changes the spacing of the capacitive gap of large load capacitor. This approach results in large, slow, complex, and failure-prone matching network implementations.
Another approach is to configure a matching network portion for START mode and then use mechanical relays to couple additional load capacitance to supplement the START load capacitance when the network switches to RUN mode. However, this approach couples the impedance of the two different impedances, making is difficult to tune each impedance to optimize impedance matching in both modes. In addition, such an approach also results in matching networks that are too large and inefficient, require too many expensive components, and switch too slowly for some plasma applications.
Against this backdrop, the present invention has been developed. The present invention relates to an improved RF switching network employing solid-state transistors, PIN diodes, and TRIAC interfaces, or some sub-combination thereof. In one embodiment, PNP bipolar transistors and PIN diode attenuators are used to switch quickly between a matching network tuned to the START mode impedance and a matching network tuned to the RUN mode impedance. In another embodiment, a TRIAC interface is introduced between the PNP transistor and the PIN diode to isolate the PNP transistor""s collector from the high voltage generated at the input to the PIN diode under reverse-bias conditions. The isolation protects the PNP transistor collector-base junction from failure cause by a VCBO that exceeds the transistor""s collector-base breakdown voltage.
These and various other features as well as other advantages, which characterize the present invention, will be apparent from a reading of the following detailed description and a review of the associated drawings.